Shear capacity of reinforced concrete 10

[BigBakwas77]

I have worked previously in locations where British and Indian codes were in use. Both these codes consider the amount of flexural steel reinforcements (Tension as well as compression) in flexural members to determine shear capacity. Now I am working in Canada and CSA A23-04 does not utilize the value of the amount of flexural steel reinforcements while determining shear capacity. Surely one cannot assume that shear capacity of reinforced and un-reinforced sections shall be same. Am I missing something here?

 

[r13]

ACI used to include the flexural steel in shear capacity, but the effect was rather small.

 

[jayrod12]

It only takes into account the transverse reinforcing (stirrups) but neglects any longitudinal steel. The Vc portion of the shear capacity is the same regardless of reinforced or not.

 

[BigBakwas77]

@retired13
As per British and Indian codes, the effect of flexural steel on shear capacity is significant. I remember many instances where I was able to avoid increasing thickness of flat slab or footing to satisfy punching shear requirements because of additional shear capacity derived from flexural steel.

 

[ooox]

For one way shear using the detailed method in ACI318, 'pw' is used in some of the equations which accounts for longitudinal tension steel, the simplified method for one way shear does not account for any steel.

 

[r13]

I rarely used flexural reinforcement to increase shear capacity, instead, we preferred to assign a higher strength concrete, which is more reliable with minimal increases in cost.

 

[rapt]

I do not have a copy of the 2004 version, but the 2014 version definitely accounts for Ast in the calculation of the tensile strain for the calculation of the shear slope and the concrete shear capacity in the General Method. The Simplified method does not include it but makes an assumption that the Ast required for flexure + longitudinal tension force for shear is supplied.

You should read up more in Collins & Mitchell papers on the development of the methodology for the MCFT that the code is based on!

 

[MSUK90]

@BigBakwas77, @retired13, @jayrod12; I also use BS code for design and the effect of flexural reinforcement is considerable for preventing punching. I know that there are many ways to increase section capacity(as retired13 suggested to increase concrete strength) but I still would like to know why "flexural reinforcement" affect is not considered in ACI and CSA even though its that much significant as per BS and Indian codes. Would also like @KootK to join the conversation()..

 

[Settingsun]

This topic has a graph towards the end showing the trend for the Australian code which, for shear, is based on the Canadian code.

https://www.eng-tips.com/viewthread.cfm?qid=446366

When the reinforcement is heavily utilised by bending and tension, the increase in shear capacity is not much, but there is an increase for constant bending moment because increasing the reo quantity reduces its utilisation.

 

[Settingsun]

BS8110 is withdrawn I believe, and IS456 getting quite old. The test results for shear were turned into design requirements quite differently in different countries. The Canadians think they now have a better theory backed by better and more recent testing and are probably right in that regard.

CSA A23.3-04 equation 11-13 includes As and Ap, which is where the reinforcement quantity comes into shear capacity.

 

[IDS]

but I still would like to know why "flexural reinforcement" affect is not considered in ACI and CSA even though its that much significant as per BS and Indian codes.

As said in several other previous posts, flexural reinforcement is included in both the ACI and CSA codes. The link posted by steveh49 is worth a look.

The following quotes are relevant to the discussion.

From ACI318-2008 Commentary:
"R11.2.2.1 — Equation (11-5) is the basic expression for
shear strength of members without shear reinforcement.11.3
The three variables in Eq. (11-5), λ (as a measure of
concrete tensile strength), ρw, and Vud/Mu, are known to
affect shear strength, although some research data
indicate that Eq. (11-5) overestimates the influence of fc′ and
underestimates the influence of ρw and Vud/Mu. Further
information has indicated that shear strength decreases
as the overall depth of the member increases."

From the April 2020 issue of Concrete International (V42 No.4):
"The 2019 edition of the ACI 318 Code contains new
equations for calculation of concrete shear strength for
reinforced concrete members. The rationale for the
new equations is provided in Reference 2. In brief, the
changes reflect observations that concrete member shear
strength is affected by the member depth and the percentage
of flexural reinforcement.

(My bold emphasis)



Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[MSUK90]

@steveh49 and @IDS, thanks for the elaborate replies. Seems everything is clear now. Will go through some numbers sometime for comparison of affects of different codes on shear strength as per flexural reinforcement provided.